Recently, there are used a lot of optical discs such as compact discs, once write type optical discs, magneto-optical discs and phase change type optical discs as information recording media. These optical discs are generally produced by methods in which a thin metal film stack working as a recording layer or light reflection layer is formed by a sputtering method on a recording surface of a transparent resin substrate made of polycarbonate and the like on which fine patterns such as pits and grooves have been formed, and for preventing deterioration of these layers, a protective layer having a thickness of about 5 to 20 μm is formed with a ultraviolet ray curable type resin.
On the other hand, recently, widely spreading as those having enhanced recording capacity are, for example, DVD (digital video disc or digital versatile disc) produced by bonding optical disc substrates having a thickness of 0.6 mm, described in Japanese Patent Application Laid-Open (JP-A) No. 8-212597.
Further, JP-A No. 8-235638 suggests an optical disc as a high density optical disc having improved recording capacity than that of DVD and capable of recording high quality video data and the like for a long period of time. This high density optical disc is an optical disc which is obtained by forming a recording layer on a transparent or opaque substrate made of a plastic and the like then laminating an optical transmittance layer having a thickness of about 100 μm on the recording layer and to which recording light and/or reading light is directed through the optical transmittance layer. Regarding this high density optical disc, there is a suggestion, as described in JP-A No. 11-273147, of further increasing density by short wavelength recording utilizing laser ray having a wavelength of, for example, 400 nm which is shorter than that of red laser conventionally used for recording and/or reading of optical information media such as compact discs and DVD.
As methods of forming this optical transmittance layer, there are listed (1) a method in which a transparent film having a thickness of 100 μm is pasted on a recording layer using an adhesive, (2) a method in which an ultraviolet ray curable resin placed on a recording layer is pushed to spread to obtain desired thickness with a flat transparent plate made of glass and the like, and cured by irradiation with ultraviolet ray via this plate, and the plate is released to give an optical transmittance layer, (3) a method in which an ultraviolet ray curable resin is applied on a recording layer by a spin coating method, then, irradiated with ultraviolet ray to obtain an optical transmittance layer.
As the ultraviolet ray curable resin used for formation of such an optical transmittance layer, there are listed, for example, compositions described in JP-A Nos. 3-131605 and 4-264167. There can also be used ultraviolet ray curable resins for transfer of optical disc stamper, for formation on a transparent substrate of tracking grooves described in JP-A Nos. 61-208646, 62-88156 and 5-59139.
However, when an optical transmittance layer of a high density type optical disc described above is formed using ultraviolet ray curable resins described in JP-A Nos. 3-131605, 4-264167, 61-208646 and 62-88156, there is a practical problem that significant skew occurs on an optical disc since the ultraviolet ray curable resin has high volume shrinkage coefficient. With respect to a composition described in JP-A No. 5-59139, when a composition of ratio causing no skew is prepared, there occurs a problem that the resulting cured substance has poor hardness, leading to difficulty in making balance between hardness and skew. With a composition adopted in examples described in this publication, significant skew occurs though hardness is high.
JP-A No. 54-127994 describes a coating composition containing a specific amide-modified urethane acrylate radiation-curable compound. However, this coating composition is used for formation of a protective coating film of a vinyl cover, and is a composition used in the field utterly different from a coating layer of an optical disc. Further, the object of inclusion of a urethane acrylate in this coating composition is to impart bending strength and tensile strength to a cured film, and based on a viewpoint irrelevant to problems (e.g. volume shrinkage) specific to a coating layer of an optical disc as described above.